Zero-Allocation Engineering

Eliminating GC Spikes for High-Density Flocks at 60+ FPS

In mobile, console, and VR titles, steady frame pacing is non-negotiable. Even small garbage collection allocations ($24\text{ B} - 64\text{ B}$ per agent per frame) quickly accumulate across a flock of 50 creatures, triggering frequent, jarring GC collection spikes.

PERCH is engineered from the ground up for 0 B steady-state managed heap allocations.

The 5 Pillars of Zero-GC Design

+-----------------------------------------------------------------------------------+
|                        ZERO-ALLOCATION RUNTIME STRATEGIES                         |
+-----------------------------------------------------------------------------------+
| 1. Non-Alloc Physics Buffers    ===> Physics.SphereCastNonAlloc with Static Arrays|
| 2. Zero-Closure Event Queues    ===> Fixed-Size Struct Ring Buffers (QueuedEvent) |
| 3. Precomputed LUT Trajectories ===> 32-Sample Preallocated Float Arrays          |
| 4. Value-Type Identity Structs  ===> readonly struct SlotHandle, LeaseToken       |
| 5. Spatial Hash Direct Mapping  ===> Pre-Sized Hash Buckets & Direct Int Dictionaries|
+-----------------------------------------------------------------------------------+

1. Preallocated Physics Buffers (DEC-006)

Unity’s standard Physics.SphereCastAll allocates a new RaycastHit[] array on every single call. In PERCH:

All spatial sweeps, approach checks, and departure tests use Physics.SphereCastNonAlloc.
Query hit arrays are statically sized during initialization (RaycastHit[64] / RaycastHit[256]).
If surrounding geometry saturates buffer capacity, the solver safely flags QuerySaturated rather than resizing on the heap.

2. Struct Circular Ring Buffers (DEC-010)

Standard C# System.Action closures and List<T> queues generate managed heap objects whenever lambdas or dynamic array resizes occur.

Commands use a preallocated PendingCommand[16] ring buffer.
Events use a preallocated QueuedAgentEvent[32] ring buffer.
Adding, inspecting, and draining events operates purely on stack and fixed struct memory with zero heap activity.

3. Elimination of Event Closure Allocations (DEC-008)

In PerchAgent.cs, all UnityEvent invocations (OnStateChanged, OnLanded, OnDeparted, OnLandingFailed) are strictly guarded with null checks before creating invocation delegates.

4. Value-Type Data Layout

Core data structures cross subsystem boundaries as unmanaged C# readonly struct values:

SlotHandle: 16 bytes.
LeaseToken: 24 bytes.
MotionCommand: 36 bytes.
PerchSnapshot: 32 bytes. These structs live entirely on the stack, passing between methods by value or in reference without creating garbage collector pressure.

5. Pre-Allocated Arc-Length LUTs (DEC-004)

Dynamic numerical integration of spline arc lengths per frame causes significant GC churn. PERCH preallocates a fixed 32-sample float array for every trajectory candidate, reusing the buffer across evaluations.

Profiler Verification Evidence

PERCH’s zero-allocation design is verified in the Unity Profiler:

Simulation Workload	Active Agents	Authored Slots	Profiler Status
Free Flight Roaming	100 Birds	200 Slots	PASSED
Simultaneous Approaches	24 Birds	48 Slots	PASSED
Panic Wave Takeoff	50 Birds	100 Slots	PASSED
Stress Benchmark	300 Agents	600 Slots	PASSED